1. Field of the Invention
This invention relates to an AlGaAs semiconductor laser device with high reliability, and it also relates to a method for the production of such a semiconductor laser device.
2. Description of the Prior Art
There have been widely used semiconductor laser devices with a buried groove forming a current injection path, which are referred to as buried-type semiconductor laser devices. A conventional process of producing a semiconductor laser device of this type is shown in FIGS. 3A to 3C.
First, as shown in FIG. 3A, on an n-GaAs substrate 1, an n-Al.sub.0.5 Ga.sub.0.5 As first cladding layer 21, an Al.sub.0.14 Ga.sub.0.86 As active layer 3, a p-Al.sub.0.5 Ga.sub.0.5 As second cladding layer 22, and an n-GaAs current blocking layer 62 are successively formed by an epitaxial growth technique. Next, a photoresist pattern 102 with a striped opening is formed on the n-GaAs current blocking layer 62 which is then selectively etched in such a manner that the etching is allowed to stop at the surface of the p-Al.sub.0.5 Ga.sub.0.5 As second cladding layer 22, to form a striped groove as shown in FIG. 3B.
After removing the photoresist pattern 102, a p-Al.sub.0.5 Ga.sub.0.5 As third cladding layer 23 and a p-GaAs contact layer 7 are successively grown by an epitaxial growth technique, as shown in FIG. 3C. Finally, an n-side electrode 92 is formed on the bottom surface of the n-GaAs substrate 1, and a p-side electrode 91 on the top surface of the p-GaAs contact layer 7, resulting in a semiconductor laser device.
However, in this production process, the surface of the p-Al.sub.0.5 Ga.sub.0.5 As second cladding layer 22, which forms the bottom of the striped groove, is oxidized through exposure to air during the time between the etching of the n-GaAs current blocking layer 62 and the successive growth of the p-Al.sub.0.5 Ga.sub.0.5 As third cladding layer 23 and the p-GaAs contact layer 7. The layer grown by an epitaxial growth technique on the oxidized surface of the p-Al.sub.0.5 Ga.sub.0.5 As second cladding layer 22 consequently has many crystal defects in the vicinity of the interface therebetween.
Such a problem occurs if any epitaxial growth technique, such as liquid-phase epitaxy (LPE), molecular beam epitaxy (MBE), or metal-organic chemical vapor deposition (MOCVD), is used. Most of these crystal defects create deep energy states, so that when the semiconductor laser device having such crystal defects is operated, part of the generated laser light is trapped by the deep states and converted into heat, thereby causing an increase in the temperature in the vicinity of the active layer. Furthermore, these crystal defects act as non-radiative recombination centers, thereby reducing the amount of light emission and thus resulting in the deterioration of the device characteristics.
To solve these problems, there is a known method which utilizes the property of GaAs having a tendency to melt back by the LPE method. For example, an AlGaAs semiconductor laser device is produced by the process shown in FIGS. 4A to 4E.
First, as shown in FIG. 4A, on an n-GaAs substrate 1, an n-Al.sub.0.5 Ga.sub.0.5 As first cladding layer 21, an Al.sub.0.14 Ga.sub.0.86 As active layer 3, a p-Al.sub.0.5 Ga.sub.0.5 As second cladding layer 22, a p-GaAs buffer layer 4, a p-Al.sub.0.5 Ga.sub.0.5 As etching stopper layer 5, and an n-GaAs current blocking layer 62 are successively grown by an appropriate epitaxial growth technique. Next, a photoresist pattern 102 with a striped opening is formed on the n-GaAs current blocking layer 62 which is then selectively etched by an ammonia etchant in such a manner that the etching is allowed to stop at the surface of the p-Al.sub.0.5 Ga.sub.0.5 As etching stopper layer 5, to form a striped groove as shown in FIG. 4B.
After removing the photoresist pattern 102, the substrate is immersed in an aqueous solution of hydrofluoric acid to remove the portion of the p-Al.sub.0.5 Ga.sub.0.5 As etching stopper layer 5 on the bottom of the striped groove, as shown in FIG. 4C. The wafer thus processed is then inserted into an LPE apparatus in which the wafer is placed in contact with a Ga-Al-As melt to allow the surface of the n-GaAs current blocking layer 62 and the portion of the p-GaAs buffer layer 4 on the bottom of the striped groove to melt back, thereby exposing the p-Al.sub.0.5 Ga.sub.0.5 As second cladding layer 22 at the bottom of the striped groove. Then, using the same LPE apparatus, a p-AlGaAs third cladding layer 23 and a p-GaAs contact layer 7 are successively grown. Finally, an n-side electrode 92 is formed on the bottom surface of the n-GaAs substrate 1, and a p-side electrode 91 on the top surface of the p-GaAs contact layer 7, resulting in a semiconductor laser device as shown in FIG. 4D.
In the above-described melt-back process using the Ga-Al-As melt, the p-GaAs buffer layer 4 present on the bottom of the striped groove does not melt back satisfactorily, but rather, the n-GaAs current blocking layer 62 forming the shoulders of the striped groove melts back to a large extent. For example, the p-GaAs buffer layer 4 may remain with almost no melt-back, while the n-GaAs current blocking layer 62 forming the shoulders of the striped groove melts back to increase the width of the striped groove, resulting in a semiconductor laser device with a structure as shown in FIG. 4E.
Furthermore, the width of the striped groove depends on the degree of distributed supersaturation of the Ga-Al-As melt, etc., and varies from groove to groove within the same wafer. Because the p-GaAs buffer layer 4 remaining on the bottom of the striped groove absorbs the light generated from the Al.sub.0.14 Ga.sub.0.86 As active layer 3, there occurs a decrease in the intensity of light emission. Even if the p-GaAs buffer layer 4 is made extremely thin and completely melted back, there still occurs a variation in the width of the striped groove, thereby causing a variation in the threshold current and thus reducing the product yield.